Method of physical vapor deposition

ABSTRACT

A method of physical vapor deposition is described wherein a source of evaporant material and a substrate are placed in a vacuum chamber maintained at a pressure of less than 10 4 Torr. A vapor of the evaporant material is produced for deposition on the substrate. The vapor is intercepted with the beam of electrons to produce positive ions, and the substrate is maintained at a negative electrical potential sufficient to cause substantial heating of the substrate by ion bombardment.

United States Patent Kennedy Oct. 14, 1975 METHOD OF PHYSICAL VAPOR 3,504,325 3 1970 Rairden 117/933 DEPOSITION 3,751,310 8/1973 ChO 117/933 [75] Inventor: Kurt D. Kennedy, Berkeley, Calif, Primary Examiner john H. Newsome [73] Assignee: Airco, Inc., Montvale, N.J.- Attorney, Agent, or Firm-Fitch, Even, Tabin & 22 Filed: Jan. 14, 1974 Luedeka [21] Appl. No.: 432,874 ABSTRACT Application Data A method of physical vapor deposition is described commuatlon-m-pal't of 232,388, wherein a source of evaporant material and a sub- ]972 abandoned strate are placed in a vacuum chamber maintained at a pressure of less than 10 Torr. A vapor of the [52] US. Cl. 427/8; 427/13; 427/38 evaporant material is producedvfor deposition on the [51] Int. Cl. C23C 13/02; C23? 13/04 Substrate The vapor is intercepted with the beam of [58] held of Search 1 17/933 electrons to produce positive ions, and the substrate is 1 17/106 R maintained at a negative electrical potential sufficient to cause substantial heating of the substrate by ion [56] References Clted bombardment UNITED STATES PATENTS 12/1966 Gowen 117/933 4 Claims, No Drawings METHOD OF PHYSICAL VAPOR DEPOSITION This application is a continuation-in-part of application Ser. No. 282,388 filed Aug. 2i, 1972, now abandoned.

This invention relates to physical vapor deposition and, more particularly, to an improved method for physical vapor deposition under conditions of elevated substrate temperatures.

Physical vapor deposition of materials on substrates is of considerable advantage in many production operations. Physical vapor deposition is generally carried out in a relatively high vacuum environment in which the material to be deposited is evaporated by raising it to a sufficiently high temperature. The required heat may be produced by suitable heating, such as by electron beams.

For some types of materials, it is desirableto elevate the substrate temperature and maintain it at theelevated temperature during the physical vapor deposition process. In some cases, this is done to enhance ductility in the resulting deposit. In other cases, it is done to enhance the adhesion between the deposit and the substrate, or to achieve a desired crystalline configuration in the deposit. Typically, the substrate is heated to a temperature which is at least of its absolute melting temperature, and often, the heating is to substantially higher temperatures.

During the coating operation, a certain amount of heat input will occur into the substrate as a result of the latent heat of vaporization in the condensing vapor. At temperatures up to about 500F, the heat input from the coating itself, as a result of the latent heat of vaporization, is usually sufficient to maintain a desired temperature. Above about 500F, however, it is typically necessary to provide an additional source of heat for the substrate during the coating process.

One means of heating substrates being coated is shown and described in US. Pat. No. 3,560,252, as signed to the assignee of the present invention. The technique described in the aforesaid patent has proved to be of substantial and significant commercial importance. Certain types of substrates, however, do not readily lend themselves to heating by radiant heaters. For example, strip lines in which a continuous strip of substrate is fed into and out of a vacuum chamber may render radiant heating somewhat inconvenient.

Accordingly, it is an object of the present invention to provide an alternative means of substrate heating to the use of radiant heaters.

Another object of the invention is to provide an improved method of physical vapor deposition.

It is another object of the invention to provide an improved method of physical vapor deposition which is particularly suited to continuous strip substrates.

Other objects of the invention will become apparent to those skilled in the art from the following description.

Very generally, the method of the invention is directed to depositing an evaporant material on a substrate. A source of evaporant material and a substrate are placed in a vacuum chamber. The vacuum chamber is maintained at an ambient pressure of less than 10 Torr. A vapor of the evaporant material is produced in the vacuum chamber for deposition on the substrate. A beam of electrons is directecl atthe vapor to produce positive ions. An electrical potential is maintained on ing that is required to achieve a given temperature.

2 the substrate which is sufficient to cause substantial heating of the substrate by ion bombardment.

Referring now to the invention in greater detail, the method of the invention is practiced, preferably, in an electron beam furnace or similar apparatus. Apparatus of this general type is illustrated schematically in the previously mentioned US. patent. Also, apparatus of this general type is well known to those skilled in the art, and therefore will not be described in detail herein.

Typically, the electron beam furnace used for physical vapor deposition comprises an evacuated enclosure, means for maintaining a pressure in the enclosure (other than in the vapor cloud), hereinafter referred to as the ambient pressure, of less than 10" Torr. A vapor of the evaporant material is produced from a molten pool or by sublimation from a solid, by suitable heating. Preferably, this is accomplished by means of an electron beam directed at a solid orliquid source of the evaporant. -After the vapor cloud is produced, the cloud is allowed to intercept the substrate such that evaporant condenses on the surface of the substrate, providing the coating.

In order to produce the vapor, the target evaporant material is heated by any suitable means. Preferably, such heating is accomplished by an electron beam directed on to the surface of the evaporant material by suitable magnetic fields.

Typically, a substantial portion of the vapor cloud thus, produced is intercepted by the electron beam. The impingement of electrons upon the vapor particles produces ionization and, consequently, a large number of positive ions are present in the chamber.

As will be explained, the positive ions thus produced are utilized, in accordance with the invention, to produce heating of the substrate. To this end, the substrate ismaintained at an electrical potential sufficient to cause substantial heating of the substrate by ion bombardment. This is accomplished by adjusting the operating pressure in the system, such as by bleeding in a suitable inert gas such as Argon, and by adjusting the bias potential on the substrate, such that the heat input balances the heat output at the desired substrate temperature, thus establishing a condition of equilibrium. The heat output, of course, is principally through the mode of radiation, especially in the case of discrete substrates. Some conduction, however, will occur. For example, in the case of discrete substrates, conduction will occur through the mechanical means by which the discrete substrate is supported. Even greater heat loss through conduction will occur in the case of a moving strip, since heat is conducted along the strip both in the upstream and downstream directions. Thus, it is typically more expedient to establish the equilibrium conditions empirically through experimental set-ups, rather than attempting to calculate them in advance.

The potential may be either a high negative potential with respect to the source, or may be a high alternating current potential. In either case, the potential is preferably great enough to accelerate a suffficient number of positive ions to the substrate so as to provide a power density on the substrate of about 6 or 6 /2 watts per square inch of exposed surface. The vapor source will, of course, supply some radiant heat and therefore the larger andhotter the vapor source is, the less ion heat- The amount of potential required for accelerating sufficient ions depends upon the amount of ions available Th is' of course, depends upon the pressure in the 3 system including the amount of vapor present (i.e. the evaporation rate). Thus, in cases where material such as zinc is being evaporated, typically providing very 4 input was below the desirable minimum level and therefore provided a substrate temperature of only 340F.

For precise control of substrate heating, it may be dehigh evaporation rates, only 1 KV or less bias voltage will be required for the requisite heating. On the other sirable to provide a closed loop arrangement. Thus, hand, where a relatively low evaporation rate exists, suitable sensors may be employed to sense the temperbias voltages of 10 KV or greater may be required. ature of the substrate and provide an electrical signal Some lessening of the required voltage may be obrelated thereto. This electrical signal may then be utitained by raising the system pressure through use of an lized to regulate or control the bias voltage on the subinert gas bleed. strate, thereby maintaining a relatively constant sub- The following Table illustrates the effect of changes strate temperature. The circuitry required for such an in pressure on current and hence power input to the electrical arrangement will be readily apparent to those substrate in a deposition set-up. Examples 1-5 relate to skilled in the art. the heating of a stainless steel disc 1 1 inches in diame- It may therefore be seen that the invention provides ter providing a surface area of about 190 sq. inches. an improved method of physical vapor deposition in The second group of examples, 6-10, relates to the which substrate heating is accomplished by the accelercoating of a2 inch square stainless steel substrate 0.080 ation of ions present in the vapor cloud to impinge inches thick, thus providing a surface area of 4 square upon the substrate. The invention is particularly suitinches. In each case, the pressure of the system in miable for situations in which substrate heating by other crons is given, the bias voltage on the substrate (held means, such as by radiant heaters, is undesirable or imsteady) in kilovolts is given, the measurable electrical practical. current flowing into the substrate as a result of ion Various modifications of the invention in addition to bombardment is given, the approximate temperature of those shown and described herein will become apparthe substrate at equilibrium is given, and the input ent to those skilled in the art from the foregoing depower density to the substrate in watts per square inch scription. Such modifications are intended to fall within is also given. the scope of the appended claims.

Prcssurc Bias Volt. Current Pwr. lnput Subst. Temp. Ex. N0. (micron) (KV) (amps) (watts/sq.in.) F

1 50 3.5 0.8 20 650 2 40 3.5 0.76 14 5x0 3 30 3.5 0.52 9.6 550 4 20 3.5 0.36 6.6 500 5 10 3.5 0.03 5.5 300 6 3.5 0.04 35 1600 7 40 3.5 0.04 35 950 x 30 3.5 0.06 52.5 850 9 20 3.5 0.08 70 70.0 l0 l() 3.5 0.12 105 50.0

As may be seen from the Table, example 5 resulted in an equilibrium temperature below that which is commercially significant, since such a low equilibrium tem- What is claimed is: perature could be achieved by suitable adjustments in l. A method of physical vapor deposition for depositthe evaporation rate and without a substrate bias. It ing material on a substrate, comprising, placing a may also be seen that a power input density of at least source of evaporant material and a substrate in a vacabout 6 or 6%: watts per square inch is typically necesuum chamber, maintaining an ambient pressure in the sary to achieve the desired minimum level substrate vacuum chamber of less than 10 Torr, heating the temperature of about 500F. evaporant material by means of an electron beam to In the following Table, the result of variations in bias 6O produce a vapor for deposition on the substrate and involtage on both the power input density, and the subtercepting the vapor with the beam of electrons to prostrate temperature may be seen. The examples were duce positive ions, and applying an electrical potential obtained in a set-up identical to that of the previous to the substrate sufficient to cause ion bombardment of Table holding a constant system pressure of 50 microns the substrate at a power density of at least about 6 watts and varying the bias voltage in increments of 500 volts. As may be seen, corresponding increases occurred in current amperage, power input and substrate temperature. It may also be seen that in example I l, the power per square inch of exposed substrate surface and maintain the substrate at an equilibrium temperature which is at least 25% of the absolute melting temperature of the evaporant.

3 ,9 1 2,8 2 6 5 6 2. A method according to claim 1 wherein the electri- 4 A h d according t claim 1 including the addical potential maintained on the substrate is negative d-c tiona] Steps of sensing the temperature of the Substrate with respect to the source.

3. A method according to claim 1 wherein the electriand regulatmg the elecmca] potemml mamtamed cal potential maintained on the substrate is a-c with re-' 5 thereon in accordance with the sensed temperaturespect to the source. 

1. A METHOD OF HYSICAL VAPOR DEPOSITION FOR DEPOSITING MATERIAL ON A SUBSTRATE,COMPRISING, REPLACING A SOURCE OF EAPORANT MATERIAL AND A SUBSTRATE IN A VACUUM CHAMBER, MAINTINING AN AMBIENT PRESSURE IN THE VACUUM CHAMBER OF LESS THAN 10-4 TORR, HEATING THE EVPORANT MATERIAL BY MEANSOF AN ELECTRON BEAM TO PRODUCE A VAPOR FOR DEPOSITION ON THE BSTRATE AND INTERCEPTING THE VAPOR WITH THE BEAM OF ELECTRONS TO PRODUCE POSITIVE IONS, AND APPLYING AN ELECTRICAL POTENTIAL TO THE SUBSTRATE SUFFICIENT TO CAUSE ION BOMBARDMENT OF THE SUBSTRATE AT A POWER DENSITY OF AT LEAST ABOUT 6 WATTS PER SQUARE INCH OF EXPOSED SUBSTRATE SURFACE AND MAINTAIN THE SUBSTRAE AT AN EQUILIBRIUM TEMPERATURE WHICH IS AT LEAST 25% OF THE ABSOLUTE MELTING TEMPERATURE OF THE EVAPORANT.
 2. A method according to claim 1 wherein the electrical potential maintained on the substrate is negative d-c with respect to the source.
 3. A method according to claim 1 wherein the electrical potential maintained on the substrate is a-c with respect to the source.
 4. A method according to claim 1 including the additional steps of sensing the temperature of the substrate and regulating the electrical potential maintained thereon in accordance with the sensed temperature. 